Adobe has already protested the lack of Flash on the iPhone. On Friday, the company’s Flash Platform blog pledged to find ways for Flash apps to run on the iPad. The absence of Flash on the iPad was obvious during Wednesday’s debut of the device. Showing it in action, Steve Jobs surfed to the New York Times website, which had a missing Flash plug-in displayed prominently on its front door. Jobs did not mention Flash during his presentation, nor has the company provided official word on why it’s missing from the iPhone and iPad.

The issue of Flash is important because so much of the web’s rich media is encoded in Adobe’s format. Critics point out that the iPad won’t be able to play most movies on the Web – a glaring omission for such a media-centric device.

Apple is betting that HTML 5 will fill the gap.

HTML 5 is an emerging standard for rich media that promises to do away with plugins. It moves away from single vendor technologies like Flash, Silverlight and Java FX. HTML 5 is already built into Safari, Firefox and Chrome.

But although YouTube and Vimeo are already using HTML 5 for their mobile video sites, it isn’t yet widely deployed. Some experts predict HTML 5 won’t be popular for at least three years, unless the iPad breaks records, of course.

Apple would likely use HTML 5 on the iPad in conjunction with CSS (which would reformat mobile sites to fit the iPad’s 4:3 aspect ratio, rather than the iPhone’s 16:9).